A dye-sensitized solar cell has been proposed by Michael Graetzel, Switzerland, et al., and has received considerable attention as a high-conversion efficiency and low-cost photoelectric conversion element (see, for example, patent document 1 and non-patent document 1).
FIG. 7 illustrates an exemplary related art dye-sensitized solar cell in a cross-sectional view.
A dye-sensitized solar cell 100 mainly includes a first substrate 101, a second substrate 105 and an electrolyte 106. The first substrate 101 includes, on a surface thereof, a sensitizing dye-carrying porous semiconductor electrode (hereinafter, referred to as “dye-sensitized semiconductor electrode”) 103. The second substrate 105 includes, on a surface thereof, a conductive film 104. The electrolyte 106 includes an oxidation-reduction couple (redox pair), such as iodine and iodide ion, and is sealed between the first and second substrates 103 and 105.
The first substrate 101 uses a light-transmitting plate material. The first substrate 101 includes, on a surface in contact with the dye-sensitized semiconductor electrode 103, a transparent conductive layer 102 which imparts conductivity. The first substrate 101, the transparent conductive layer 102 and the dye-sensitized semiconductor electrode 103 altogether constitute a working electrode (i.e., a window electrode) 108.
The second substrate 105 includes, on a surface in contact with the electrolyte 106, a conductive film 104 comprising, for example, of carbon or platinum, which imparts conductivity. The second substrate 105 and the conductive film 104 altogether constitute a counter electrode 109.
In the dye-sensitized solar cell 100, the first substrate 101 and the second substrate 105 are positioned at a predetermined distance from each other with the dye-sensitized semiconductor electrode 103 and the conductive film 104 opposing each other. A sealant 107, which may be thermoplastic resin, is provided between, and at the periphery of, these substrates. The first substrate 101 and the second substrate 105 are bonded together with the sealant 107 to provide a stacked cell. A space between the electrodes 108 and 109 is filled with an organic electrolyte containing the oxidation-reduction couple, such as iodine and iodide ion, introduced via an electrolyte inlet 110 to provide a charge transfer electrolyte 106.
The dye-sensitized solar cell 100 should possess visible light transmittance and high conductivity especially at the light-incident electrode thereof (i.e., the window electrode 108). The window electrode 108 has therefore been a conductive substrate that includes a transparent conductive metal oxide, such as indium tin oxide (ITO) or fluorine-doped tin oxide (FTO), applied onto the first substrate 101 (e.g., a glass substrate or a plastic substrate).
However, the substances used in the conductive substrate, such as indium (In), are rare metal which are becoming increasingly expensive and thus are preventing cost reduction of the photoelectric conversion element. It is therefore desired, from the viewpoint of significant reduction in cost, to develop a dye-sensitized photoelectric conversion element that eliminates the need of a conductive substrate and requires a reduced amount of rare metal while keeping the light-receiving efficiency.